At the outbreak of the Mexican War in 1846, the U.S. Army was sent to occupy the New Mexico territory that included all of Arizona and a portion of Nevada.
Indian wars, interrupted by the American Civil War, defined the next forty years of New Mexico’s military history.
The New Mexico territory, which included New Mexico, Arizona, and the southern part of Nevada, played a role in the Trans- Mississippi Theater of the American Civil War.
In 1861, the Confederacy waged the ambitious New Mexico campaign in an attempt to control the American Southwest and to open up access to the Union of California. Confederate power in the New Mexico territory was effectively broken after the Battle of Glorieta Pass in 1862.